This disclosure relates to methods for pre-press color match verification and correction, with special emphasis on limiting the user involvement in said process by incorporating an intelligent pre-filtering to judge color match relevance. Color management provides the tools to reconcile the different color capabilities of monitors, scanners, printers and printing presses to ensure consistent color throughout the production process. Color management also allows digital proofing, which is especially important now that the average run-length of print jobs is continuously declining. Color management is based on defined and standardized color spaces. The range of colors, or gamut, that can be captured on film, displayed on a computer monitor, or rendered by a printer vary significantly. Additionally, the range of colors that can normally be perceived by the human eye in general exceeds the colors that can be presented by the different devices.
The different attributes of the different color devices also have led to different mathematical descriptions of the individual color spaces, with two major color space classes being additive and subtractive color spaces. RGB is an additive color space that combines red, green and blue light to create all other colors. RGB color spaces are used by monitors, digital camera and scanners. CMY and CMYK color spaces as are used in most printing processes, on the other hand, and are subtractive color spaces using cyan, magenta, yellow and optionally black inks on paper to absorb red, green and blue light. The remaining reflected light is the color perceived by the viewer.
When transferring color descriptions from one device to another, ICC (International Color Consortium) profiles are frequently used. An ICC profile is a computer file that describes the mathematical transformation of one color space to another. For efficiency reasons, this is generally done through an intermediate connection space (named a Profile Connection Space PCS), resulting in ICC profiles for input devices giving the mathematical description of the input device color to the connection space and ICC profiles for output devices giving the mathematical description of the connection space to the output device. A color engine (also called a color matching module or CMM), reads the ICC profiles and converts the colors between the different devices. Multiple input descriptions, when converted to a single output device, can result in the identical color, i.e., one color might be described in a multitude of mathematically different color spaces on the input side. After correct mapping to the single output color space these different color descriptions will again describe the same color, as seen by a human.
Most electronic documents to be printed or output on a particular device include multiple elements, such as text, photos, graphics and the like. Many electronic documents are also a composite of other smaller documents and elements. For example, photos may be pasted into a largely text document at different locations. Color graphics and monochrome images may occur on the same page. The individual elements of an electronic document that are intended to match in color may be represented in a variety of color spaces, a situation which for example may arise because those elements are derived from prior documents of differing origins. This situation may not be immediately apparent to the user, because the colors of the objects appear to match on the display or when printed using a straightforward color transformation process, such as is typical in ICC-based color management.
One problem arises when more sophisticated object optimized color transformation is involved, such that different source color definitions take different color transformation paths. For example, two different objects of identical color—potentially described in different forms and different color spaces—might be transformed by the CMM to optimally use the device capability, incorporating typical device compromises as the well known compromise between color and detail preservation, or the well known compromise between color precision and color representation texture. Additionally, if one object's color is specified in sRGB for instance, and another object's color specified in SWOP CMYK (Specifications for Web Offset Publications CMYK), the color processing needed to produce the device CMYK for the specific marking process may produce a different device CMYK quadruplet for the two objects. This can be understood because all 3-to-4 transformations, such as the one from sRGB to CMYK, are underdetermined and additional degrees of freedom can be incorporated. Examples are differing black generation/black preservation strategies, even if the two objects would have matched exactly on a SWOP press using the standard SWOP assumptions.
Another problem arises when more sophisticated color transformation is involved, such that non-color differences in the source (e.g., differing object type) that cause the objects to take different color transformation paths. One instance of this is that the Xerox Corporation DocuSP® printer color DFE (digital front end) can assign different ICC rendering intents to different object types. For example, by default text is assigned a “Saturation” rendering intent, while graphics are assigned a “Relative Colorimetric” rendering intent. Especially for colors near the edge or outside the printer's color gamut, the processing of the source color may produce visibly different results. This type of processing is commonly referred to as Object Optimized Rendering (OOR) and is intended to optimally utilize the machine capabilities and trade-offs. Consider, for example, a print job consisting of three characters printed in a first color on a white background adjacent to three characters printed in white on a background of the first color. Using a program such as Adobe® Acrobat® software, the first color would be displayed on a monitor for a user to view. When the job is printed using the DocuSP® printer's color processing, the first color is slightly different.
These situations are difficult to find prior to printing without a very detailed and precise understanding of the DFE's color processing, which the typical user does not have. Consequently, color matching differences of these types have typically been discovered only upon printing, when resolution is costly and time-consuming. What is needed is a method which alerts a user to color mismatch problems prior to printing and allows the user to correct any mismatches. In some cases it may be appropriate to change the “problem” criteria described above to better reflect human perceived relevance thereby reducing the number of objects examined and to add additional relevance criteria that are based on other object properties than the described and measured color properties. Also, what is needed is a way to prioritize the visual importance of the color mismatches along with a resolution approach that can resolve the different cases in the respectively appropriate manner.
Proofing is a convenient means of providing a user, especially a remote user of a preview of how a particular print job will appear when printed on the selected printer. Currently, there are no color transformation utilities that mimic a DFE's color architecture remotely, i.e., independently of the production rip. Proofing color transforms are typically performed via an output ICC profile for a target printer used as a CMYK source profile on the DFE driving a proofer. Unfortunately, the target ICC profile can only characterize one print condition for CMYK image objects, and fails to correctly model any sophistication in the color processing of the target printer beyond the simple ICC workflow. What is needed is a remote proofing method for displaying a proof of a print job that reflects the remote printer's architecture.